During Phase I, Innovation Associates demonstrated that it is feasible to develop a fiber optic immuno-sensor based upon total internal reflection fluorescence (TIRF) which can identify the fertile period. It does this by quantitative immunoassay on an optical fiber, of urinary estrone-3- glucuronide (E1-g) and pregnanediol glucuronide (Pd-g). E1-g begins rising around 5 days prior to the LH surge and peaks 3 days prior to ovulation; and Pd-g rises following ovulation. Taken together they can accurately describe the timing of events during the fertile period with greater precision than can be obtained with a luteinizing hormone (LH) measurement. This is very valuable information for women undergoing in vitro fertilization, where optimum timing of procedures is better reflected by the E1-g peak than by the LH peak. The gradual rise of the E1-g peak, which poses difficulties in developing an on/off color test, poses no problem for the TIRF biosensor. During Phase II, Innovation Associates proposes to develop a prototype fertility measurement device suitable for use in a physicians office or fertility clinic. Eventually, as the technology becomes less expensive, the instrument will provide a reliable aid to natural family planning.